PROJECT SUMMARY Obstructive sleep apnea (OSA) is a common condition associated with significant adverse health outcomes. An estimated 25% of men and 10% of women will have OSA during their lifetime. OSA is associated with an increased prevalence of insulin resistance and type 2 diabetes and, with severe degrees of OSA, non-alcoholic fatty liver disease (NAFLD) as well. The mechanisms accounting for the association between insulin resistance and OSA are not fully understood. We have previously demonstrated that experimentally-induced sleep restriction in healthy volunteers led to a reduction in whole-body insulin sensitivity and increased rates of lipolysis and gluconeogenesis, accompanied by an increase in stress hormone levels. Studies by others suggest that, in animal models studied under hypoxic conditions, hepatic carbohydrate and lipid homeostasis are perturbed leading to hepatic steatosis and inflammation. Taken together, these observations form the basis of our overarching hypothesis that patients with OSA and hypoxia (H-OSA) have greater degrees of insulin resistance in both liver and adipose tissue when compared to those without hypoxia (NH-OSA) thus leading to increased risk for the development of diabetes in the former group. In Aim 1 we will test the hypothesis that, although individuals with OSA have been shown to have insulin resistance in multiple target tissues (adipose, muscle, liver, beta cell), these abnormalities will be significantly greater in patients with OSA that is accompanied by hypoxia (H-OSA), in comparison to those without hypoxia (NH-OSA). We will compare tissue-specific insulin sensitivity in 30 subjects with H-OSA and 30 with NH-OSA matched for sex, ethnicity, age, BMI, and apnea-hypopnea index. Hepatic and extra-hepatic insulin sensitivity will be measured using hyperinsulinemic-euglycemic clamps and stable isotope tracer studies of endogenous glucose production, gluconeogenesis, de novo lipogenesis (DNL), and lipolysis. Beta cell function and insulin kinetics will be assessed from insulin and C-peptide concentrations during an oral glucose tolerance test. Liver fat will be measured by magnetic resonance and total lean and fat mass by dual-energy X-ray absorptiometry. In Aim 2 we will test the hypothesis that treatment with continuous positive airway pressure (CPAP) will improve insulin sensitivity in all of the target tissues and that these improvements will be greater in those with hypoxia at baseline. After stabilization on CPAP therapy and maintenance for six weeks, each of the individuals studied in Aim 1 will undergo a repeat sleep study and metabolic assessments identical to those described above in Aim 1. We speculate that in NH-OSA insulin resistance is primarily triggered by increased levels of stress hormones due to fragmented sleep and this is manifested largely in extra-hepatic tissues (muscle and adipose), whereas in H-OSA there is additional stimulation of hepatic DNL, leading to liver fat accumulation and hepatic insulin resistance. .